Quantum dots (QDs) continue to intrigue technologists with the potential benefits of zero-dimensionality, low threshold current density and temperature sensitivity in modern semiconductor laser applications. The QDs formed by strain-driven processes are especially interesting since they can be easily embedded in a solid-state region to enable current injection and electrical/optical confinement. Active devices, such as, a laser, detector, modulator, etc. can be formed with a QD active region.
Problems arise for the QD active region due to its very low modal gain at the ground state energy level. Generally, for an active device to achieve high ground state modal gain requires a QD active region having a high density of states and a large overlap with the optical mode of the active device. One conventional technique for achieving high modal gain is to use stacked QD active regions. The stacked QD active regions typically include one QD active region stacked upon another QD active region and so on. The stacked QD active regions have been shown to increase ground state modal gain, which results in low threshold ground state lasing and high characteristic temperature in comparison to quantum well active regions.
However, this conventional technique has drawbacks and disadvantages. For example, one drawback is caused by the vertically propagating strain field that originates at the first QD active region and grows with each subsequent QD active region. In fact, the strain field from the first QD active region seeds the nucleation of the following QD active region, and so on, especially for the case when the QD active regions are stacked with a QD interlayer separation of less than 40 nm. Although such strain field is the cause of the columnar growth mode characteristic for all strain-coupled QD active regions, the strain energy in the strain field eventually grows too large to be absorbed by the QD formation. The strain energy may then drive defect formation such as coalescence of QDs, thus limiting the number of stacks.
A conventional solution to reduce this effect of the vertical strain field and increase the number of QD stacks is to increase the QD interlayer separation to, for example, higher than 40 nm. In this case, the vertically propagated strain fields can be diffused. However, this solution also has drawbacks and disadvantages. For example, the large interlayer separation reduces the overlap between the stacked QD active regions and the optical mode of the active device.
Thus, there is a need to overcome these and other problems of the prior art and to provide a quantum dot active structure for providing increased overlaps between the stacked QD active regions and the optical mode of the active device thus providing increased optical modal gain.